1. Technical Field
The present invention relates to membrane electrode assemblies for polymer electrolyte fuel cells, and in particular, relates to a technology which can reduce leaching of a catalyst material at the anode side during operation at high potential.
2. Background Art
A polymer electrolyte fuel cell (hereinafter simply referred to as a “fuel cell”) is formed by laminating separators at both sides of a flat electrode structure. The electrode structure is a stacked element having a polymer electrolyte membrane held between a positive side electrode catalyst layer and a negative side electrode catalyst layer, with a gas diffusion layer laminated outside of each electrode catalytic layer. In such a fuel cell, for example, when hydrogen gas is supplied in a gas passage of the separator disposed at the negative electrode side, and an oxidizing gas is supplied in a gas passage of the separator disposed at the positive electrode side, an electrochemical reaction occurs, generating an electric current. During operation of the fuel cell, the gas diffusion layer transmits the electrons generated by the electrochemical reaction between the electrode catalytic layer and the separator, and diffuses the fuel gas and the oxidizing gas at the same time. The negative side electrode catalytic layer induces a chemical reaction in the fuel gas to generate protons (H+) and electrons, and the positive side electrode catalytic layer generates water from oxygen, protons, and electrons, and the electrolyte membrane transmits protons by ionic conduction. As a result, electric power is provided from the positive and negative electrode catalytic layers.
As an electrode catalytic layer, a mixture of carbon particles carrying a catalyst material such as a rare metal and an electrolyte consisting of an ion conducting polymer, is well known. Platinum (hereinafter simply referred to as “Pt”) alone, or Pt with other metals, is known as a catalyst material (see Japanese Unexamined Patent Application Publication No. 2000-243404). In particular, a platinum-ruthenium (hereinafter simply referred to as “Pt—Ru”) alloy is used in a fuel electrode (anode) to prevent the Pt from being poisoned by impurities such as carbon monoxide contained in the hydrogen gas which is used as a fuel gas (see Japanese Unexamined Patent Application Publication No. 11-250918).
However, during operation of the fuel cell, particularly during operation at high potential, Ru in the anode may be eluted and precipitated, and gas diffusion properties may be deteriorated thereby.